This invention relates to a separator for use between an electrode and a membrane in an electrochemical membrane cell. More particularly the present invention relates to the coating and the process used to treat the polymer surfaces of the separator to make it hydrophilic.
Many unsuccessful attempts have been made at developing a process for manufacturing alkali metal hydrosulfites, such as sodium hydrosulfite or potassium hydrosulfite, electrochemically that can compete commercially with conventional zinc reduction processes using either sodium amalgam or metallic iron. The electrochemical process for making hydrosulfite involves the reduction of bisulfite ions to hydrosulfite ions. For this process to be economical, current densities must be employed in a cell which are capable of producing concentrated hydrosulfite solutions at high current efficiencies.
Control of undesirable electrochemical byproduct thiosulfate formation in hydrosulfite produced for use in the paper and pulp industry as a strong reducing agent or bleaching agent is essential. At the high concentrations of hydrosulfite previously produced by the electrochemical route, however, this byproduct reaction is more difficult to control.
Additionally, prior eleotzochemical routes to hydrosulfite have produced aqueous solutions which are unstable and decompose at a rapid rate. This high decomposition rate of hydrosulfite appears to increase as the pH decreases or the reaction temperature increases. One approach to control the decomposition rate is to decrease the residence time of the solution in the cell and to maintain the current density as high as possible up to a critical current density above which secondary reactions will occur due to polarization of the cathode.
Some of the processes of the prior art, which claim to make hydrosulfite salts electrochemically, require the use of water-miscible organic solvents, such as methanol, to reduce the solubility of the hydrosulfite and prevent its decomposition inside the cell. The costly recovery of the methanol and hydrosulfite makes this route uneconomical.
The use of zinc as a stabilizing agent for hydrosulfites in electrochemical processes has also been reported, but because of environmental considerations, this is no longer commercially practical or desirable.
More recently, U.S. Pat. No. 4,144,146 issued Mar. 13, 1979 to B. Leutner et al describes an electrochemical process for producing hydrosulfite solutions in an electrolytic membrane cell. The process employs high circulation rates for the catholyte which is passed through an inlet in the bottom of the cell and removed at the top of the cell to provide for the advantageous removal of gases produced during the reaction. Catholyte flow over the surface of the cathodes is maintained at a rate of at least 1 cm per second and the cathode is formed of fibrous mats of compressed sintered fibers with a mesh spacing of 5 mm or less. The process is described as producing concentrated solutions of alkali metal hydrosulfites at commercially viable current densities; however, the cell voltages required are high, being in the range of 5 to 10 volts. This results in excessive energy consumption. There is no indication of the concentrations of thiosulfate impurity in the product solutions.
The availability of electrodes with a high mass transfer surface area having a high surface area to volume ratio and sufficient porosity have limited the development of a commercially practical electrochemical cell design for the production of aqueous solutions of alkali metal sulfites with low concentrations of alkali metal thiosulfates as impurities. The electrodes must also seal well, be uniformly spaced apart, easily assembled and not contribute to the production of thiosulfate impurities.
These and other problems are solved in the design of the present electrolytic cell which employs an improved hydrophilic separator between the anode and the membrane in the cell to produce an alkali metal hydrosulfite. The present invention makes the surfaces hydrophilic without the traditional addition of a surfactant to the electrolyte or coating the separator's polymeric surfaces with a surface active agent that is susceptible to being easily washed off of the surfaces.